The present invention relates to a device for propelling a marine craft with hydrojet reactive forces.
An object of the invention is to provide a jet propulsion apparatus in which thrust is augmented by suitable shaping or a pump section in order to accelerate water with reduced turbulence and by suitably shaping a discharge section in order to straighten water flow without introducing penalizing perturbations in the flow.
In the prior art, as illustrated by U.S. Pat. No. 3,233,573, G.B. Patent No. 1,011,203 or French Patent No. 1,555,257, water is accelerated in a more or less complex pump section. At a discharge end of this pump section, the velocity of the water has three components which are respectively axially, tangentially and radially oriented. When leaving the pump section, the water flow enters a discharge chamber in which it is straightened by means of vanes. The flow is deflected and restricted in order that its velocity be augmented when it leaves a single discharge opening of the chamber.
These deflections and restrictions are disadvantageous to thrust efficiency because they cause frictional loss and turbulences, so that pressure consequently increases in the fluid. This increasing pressure involves further frictional losses upstream which penalize performance of the device.